Tunable laser in a sensitized transparent material including an internal resonator and optical guide

ABSTRACT

There is disclosed a compact, narrow-band single frequency laser employing a pair of internal grating reflectors photo-induced in a suitably sensitized transparent material doped with an active medium. Specifically, a laser is obtained in a photosensitized sample of dye-doped poly(methyl methacrylate). It includes an internal resonator formed by a pair of grating reflectors permanently induced in the sample, in which optical guides can also be induced upon exposure to ultraviolet radiation. The resonated laser frequency is selected from the usual broad dye emission spectrum by choice of the grating spacings. Tunability can be achieved by applying stress to the sample to change the grating spacings and the laser resonator length either simultaneously or independently.

Ullllled Stall Kaminow et al.

[54] TUNABLE LASER IN A SENSITIZED TRANSPARENT MATERIAL INCLUDING ANINTERNAL RESONATOR AND OPTICAL GUIDE [72] Inventors: Ivan Paul Kaminow,New Shrewsbury; Heinz Paul Weber, Middletown, both of [73] Assignee:Bell Telephone Laboratories, Incorporated,

Murray Hill, Berkeley Heights, NJ.

[22] Filed: April 27, 1971 [21] Appl. No.: 137,866

Softer et al., Continuously Tunable Narrow-Band Organic i YWN 51 July 4,1972' Peterson et al., Stimulated Emission from FlashIamp-Excitd OrganicDyes in Polymethyl Methacrylate. Appl. Phys. Let. Vol. I2, No. 7, (AprilI, I968) pp. 238-240.

Tomlinson et al., Photoinduced Refractive Index inPoly(Methylmethacrylate) and its Applications. Appl. Phys Let. Vol. 16,No. 12 (June 15, I97 PP- 486-489.

Primary ExaminerWilliam L. Sikes Attorney-R. J. Guenther and Arthur J.Torsiglieri [5 7] ABSTRACT There is disclosed a compact, narrow-bandsingle frequency laser employing a pair of internal grating reflectorsphoto-induced in a suitably sensitized transparent material doped withan active medium. Specifically, a laser is obtained in a photosensitizedsample of dye-doped poly(methyl methacrylate). It includes an internalresonator formed by a pair of grating reflectors pennanently induced inthe sample, in which optical guides can also be induced upon exposure toultraviolet radiation. The resonated laser frequency is selected fromthe usual broad dye emission spectrum by choice of the grating spacings.Tunability can be achieved by applying stress to the sample to changethe grating spacings and the laser resonator length eithersimultaneously or independently.

Dye Lasers. Appl. Phys. Let., Vol. 10, (May 15, 1967) pp. 12 Claims,5Drawing Figures 266-267.

STRESS APPLICATOR PUMP LIGHT 57 SOURCE 5| i 7m mi I I II IIIIUTILIZATION 58 APPARATUS DYE DOPED STRESS 56 Q JpL APPLICATORnewest/94.x

Patented July 4, 1972 3 Shun-Shut 1 FIG.

I UTILIZATION APPAPGATUS DYE- DOPED SENSITIZED SAMPLE I3 1. P. low/Nowh. F. WEBER UV RADIATION SOURCE PUMP LIGHT SOURCE DYE- DOPED SENSIT IZEDSAMPLE 23 I /Nl EN7'ORS ATTORNEY Patented July 4, 1972 3,675,157

3 Sheets-Sheet 5 FIG. 4

DYE- DOPED SENSITIZED 5 SAMPLE 42 43 I 44 I UTILIZATION PUMP LIGHTAPPQIZATUS SOURCE E PLANES OF INCREASED INDEx INCREASED INDEX OPTICALGUIDE FIG. 5 STRESS APPLICATOR PUMP LIGHT 1 l l 57 SOURCE 5| l I I l J5g UT LIZ T N t I I A IO 58 APPARATUS DYE DOPED 56 STRESS g zgtAPPLICATOR TUNABLE LASER IN A SENSITIZED TRANSPARENT MATERIAL INCLUDINGAN INTERNAL RESONATOR AND OPTICAL GUIDE BACKGROUND OF THE INVENTION Thisinvention relates to lasers and, more particularly, to dye lasersincluding internal resonators and optical guides.

In general, lasers consist of active media which produce gain whenexcited, and resonator structures which provide feedback for thestimulated emission of the radiation. The active media may be any of anumber of types of forms. While research in the laser art has producednumerous proposals for laser media, the problems encountered in thedevelopment and use of the media in operating systems have also beennumerous. For example, lasers using solid state active media are oftenquite difficult to fabricate to the degree of precision required forstable operation, and hence are quite expensive. Lasers employing liquidor gaseous active media can require relatively large, complex anddelicate arrangements for containing, exciting, or flowing the mediaduring operation. Moreover, the laser media are usually to someextent'sensitive to operating conditions such as temperature.

The most common laser resonator structure is, perhaps, a pair of mirrorsor diffraction gratings, one at each end of the active region of thelaser, which reflect the radiation back and forth through the activemedia. In most cases, the alignment of the reflectors and their spacingis critical. The problems of maintaining alignment and spacing underoperating conditions can give rise to complex and expensivearrangements.

SUMMARY OF THE INVENTION We have discovered a way to employ waveguideeffects to alleviate these problems.

In the copending application of E. A. Chandross, R. L. Fork, 1. P.Kaminow and W. J. Tomlinson Ill, Ser. No. 21,102, filed Mar. 19, 1970,and assigned to the assignee hereof, it was shown that permanent regionsof elevated refractive index could be photo-induced in properly preparedtransparent materials such as poly(methyl methacrylate) to produce highquality, three-dimensional grating reflectors and optical guides.

We have found that regions of elevated index of refraction can beproduced in sensitized transparent materials doped with a sufiicientconcentration of active medium to be suitable for lasers. Using a pairof grating reflectors photo-induced inside a sample of the material, wehave produced a simply and inexpensively constructed, compact andrugged, narrow-band, single frequency laser.

Specifically, a laser according to our invention, is obtained in acigarette-size sensitized sample of dye-doped poly( methylmethacrylate). It includes an internal resonator formed by a pair ofaligned grating reflectors permanently induced in the sample uponexposure to ultraviolet radiation. The grating spacing d between planesof equivalent index is chosen to reflect, and hence resonate, a selectedwavelength A in the usually broad dye emission spectrum.

In this arrangement, the laser can be efficiently pumped either axiallythrough the aligned reflectors by a second laser, or transversely in theregion between the reflectors by a simple flash lamp. In either case, anarrow (-0.l Angstroms) optical line at A equal to approximately 5,700Angstroms can be obtained.

One feature of the above-described laser is that the output wavelength Ais relatively insensitive to temperature change. This feature, combinedwith the compact size and the internal resonator which is permanentlyaligned, make a laser according to our invention particularly attractiveas a rugged, easy to handle, fixed frequency source useful under variousoperating conditions.

In one embodiment of our invention, an internal axial opti cal guide iscombined with the internal grating reflectors to produce a guided lasersource suitable for an integrated optics arrangement. The guidetypically confines both the pumping radiation and the selectedwavelength radiation to a small region in the laser sample providingrelatively low thresholds and high efficiencies for the laser.

Tunability is achieved in another embodiment of the invention byapplying stress to a laser sample to change the grating spacing d of thereflectors and hence to tune the resonated wavelength A Stress may alsobe employed to tune the laser resonator length.

As an inexpensive, highly monochromatic, tunable optical source, a laseraccording to our invention should be useful in many diverse fields andapplications. The laser, for example, could readily be used in systemsfor testing gases, such as air or automobile exhaust, for specificpollutants on a routine basis. Moreover, hundreds of the laser sampleswith various grating spacings and hence various output wavelengths couldbe simple and cheaply manufactured and used to replace existing samplesin operating systems as they become depleted or when a change in outputwavelength is desirable.

BRIEF DESCRIPTION OF THE DRAWING A more complete understanding of theforegoing and other features and advantages according to our inventioncan be obtained from the following detailed description with referenceto the accompanying drawings in which:

FIG. 1 is a partially pictorial, partially diagrammatic illustration ofa laser apparatus according to our invention including a laser samplewith internal grating reflectors;

FIG. 2 is a partially pictorial, partially diagrammatic illustration ofapparatus for exposing a laser sample according to our invention to makea grating reflector;

FIG. 3 is a graph of the experimental output results obtained from alaser according to our invention;

FIG. 4 is a partially pictorial, partially diagrammatic illustration ofa laser apparatus according to our invention employing an internal axialoptical guide; and

FIG. 5 is a diagrammatic illustration of apparatus according to ourinvention including means for applying stress to a laser sample to tunethe selected resonated wavelength A DESCRIPTION OF THE ILLUSTRATIVEEMBODIMENTS In FIG. 1, source 11 supplies pumping radiation to excitelaser 12 including a sample 13 of optically transparent materialsuitably sensitized for photo-induced irreversible molecular structuralchange tending to increase the refractive index thereof and doped withan active medium providing substantial gain in a particular wavelengthrange. Internal grating reflector 14 is made reflective of radiation ata selected wavelength in the gain range of the active medium andtransmissive of the pumping radiation. Grating reflector 15 is partiallyreflective at the selected wavelength and partially transmissive.Together reflectors 14 and 15 bidirectionally reflect the selectedwavelength radiation and thereby form a resonator internal to sample 13suitable for stimulating coherent emission of the radiation, a portionof which is extracted for utilization in apparatus 16.

Sample 13 is illustratively composed of poly(methyl methacrylate)sensitized for refractive index increases upon exposure to ultravioletradiation and doped with an active dye medium. The sample is typicallyprepared by first dissolving the dye of appropriate concentration in themonomer methyl methacrylate which includes sensitizing ingredients suchas peroxides. The monomer is devoid of any additives to stabilize thepoly(methyl methacrylate) against ultraviolet damage. The solution isthen polymerized at low temperatures around 40 to 50 centigrade so asnot to impair the sensitivity provided by the peroxides.

The method of obtaining the sensitized materials, excluding the activemedium, is explained in considerable detail in the above-mentionedcopending application of E. A. Chandross et al. We have applied thatmethod to materials doped with a sufficient concentration of activemedium to be suitable for lasers according to our invention.

Grating reflectors l4 and are produced in a suitably sized sample of thedye-doped photosensitized material by creating a series of partiallyreflecting parallel planes of alternating high and low refractive indexwith spacing d between equivalent planes to reinforce reflections at theselected wavelength. The reflectors are analogous to layered dielectricmirrors and should be large enough in the transverse plane to interceptmost of the radiation emitted in the sample. The means for accomplishingthe required index change in poly(methyl methacrylate) is also describedin the abovementioned copending application. Two ultraviolet beams,which are derived from the same source 21 by beam splitter 22 andreflected by mirrors 24 and 25, intersect at an angle and are incidenton a broad surface of sample 23 as shown in FIG. 2. The interplanarspacing is governed by the Bragg formula:

d M2 sin 0 where A is the wavelength of the irradiating beams fromsource 21 measured outside sample 23, and 0 is also measured outsidesample 23 from a perpendicular to the surface thereof. Source 21 isillustratively a helium-cadmium laser supplying radiation beams at A3,250 A in the ultraviolet.

The angle 6 is chosen to form a reflecting grating for normal incidenceof a beam of radiation at the selected laser output wavelength A givenby the equation A 2nd (2) where A is measured outside sample 23, n isthe refractive index of the sample 23, and d is the grating spacing.After exposing the first grating reflector, the sample 23 is translatedin a straight line by a distance L, the desired laser resonator length,and the second grating reflector is exposed. Altematively, both gratingscould be produced simultaneously by suitably masking a broad pair ofintersecting ultraviolet beams. Reflectors l4 and 15 need not beidentical in reflectivity and bandwidth.

While the exposure of the sample to the ultraviolet radiation iseffective to induce the index change, the amount and gradient of theindex change may be subsequently substantially increased by aging.Preferably, the samples are set aside until a stable condition isachieved before testing as a laser. Good results are obtained by leavingthe exposed samples in a dark place at a temperature below about 60centigrade for a period of several days. The aging has no noticeableeffect on the concentration of the active medium.

Suitable starting materials for the laser sample 13 of FIG. 1 shouldinclude a large number of solids which are useful for opticaltransmission and susceptible to molecular structural change in index ofrefraction. All of the materials suitable for the above-mentionedcopending application should be suitable for our invention provided theycan be properly doped with the active medium without substantiallyaffecting their photosensitivity. We suggest plastics such aspoly(methyl methacrylate) because of their several useful properties,some of which are described hereinbelow. In some substances, radiationat wavelengths outside the ultraviolet band may also be able to inducethe index change.

Numerous active media may be suitable as gain media for the laseraccording to our invention. The active media preferably providesubstantial gain in a relatively broad output spectrum 100A or more)andundergo little damage upon exposure to the radiation used to effect theindex change in the sample, i.e., ultraviolet radiation. Organic dyessuitable for our invention include Rhodamine 6G, Rhodamine B, and otherxanthene dyes, or coumarin, carbocyanine and rare earth chelate dyes.

Sample 13, when finally prepared and ready for use as a laser, may beexcited by passing the pumping beam from source 11 through the alignedgrating reflectors 14 and 15 as shown in FIG. 1 or, in the alternative,by a flash lamp disposed parallel and coupled to the sample by asuitable light pipe or reflector arrangement. The latter excitationmeans is especially useful in systems requiring a rugged, inexpensive,easyto-handle optical source.

It is well known that dye molecules are subject to permanent bleachingafier being excited a large but finite number of times. In general, thenumber of output pulses N, obtainable from a dye-doped sample is givenby the equation:

where N is the bleaching number for the active dye molecules, morespecifically, the numbers of transitions to the excited state a dyemolecule will undergo before permanent bleaching, N is the concentrationof the dye in the sample in molecules per unit volume, Vis the volume ofthe sample, and N is the number of pump photons per pump pulse absorbedin the volume V. Typically, the bleaching number N, for Rhodamine 6G ina solid poly(methyl methacrylate) matrix is of the order of 10.

Permanent bleaching, therefore, will eventually limit the life of a dyelaser. However, the millions of pulses obtainable from a cigarette-sizelaser sample according to our invention are more than adequate fornumerous uses and arrangements of the laser. Furthermore, there is noapparent reason why the bleaching of the dye cannot be eventuallyovercome or why other active media suitable for our invention with muchlarger bleaching numbers (N, cannot be found.

In a preliminary experiment, we constructed and operated a laser inphotosensitized poly(methyl methacrylate) doped with a Rhodamine 6G dyemedium. The laser was pumped by radiation from a frequency-doubled Nd:glass laser source at 5,300 A having a line width of approximately 30 cmand a pulse duration 1- of about 20 nanoseconds. The absorption peak ofRhodamine 6G dye in the poly( methyl methacrylate) medium is close to5,300 A.

A 38 X 10 X 4 cubic millimeter sample of the poly(methyl methacrylate)was prepared as described above with the dye concentration beingadjusted to approximately 5 X 10" molecules per cubic centimeter, or 8 X10' moles per liter, to give a small signal absorption of about 20 dBper centimeter at the 5,300 A. The absorption by the dye was found to beunchanged after polymerization. It was also found that percent of thepump power was absorbed in the sample with this concentration.

' Grating reflectors were produced in the sample by intersecting twoultraviolet beams at 3,250 A as shown in FIG. 2, having beam diametersof about 2 mm, so that the gratings occupied a volume of approximately 2X 2 X 2 cubic millimeters in the sample. After the first grating wasexposed, the poly( methyl methacrylate) sample was translated by adistance L 20 mm and the second grating was exposed. The exposure timefor each grating reflector was 2.5 minutes with a power of about 0.7milliwatts per beam. Upon completion of the exposure, the sample was setaside to age. After the passage of a couple of days, the sample wasfound to be in a stable condition suitable for testing.

Although the Rhodamine 6G dye medium absorbs weakly at a wavelength of3,250 A, the dye had a negligible effect on the photodielectricsensitivity at the concentration employed. The index change An betweenhigh and low index planes in the gratings tuned for reflecting awavelength A 5,702 A was approximately 10". The gratings reflectivity 1;was about 50 percent, and the grating bandwidth A) was approximately 0.4A. This bandwidth would have been decreased by increasing the effectivenumber of reflecting planes in the gratings, i.e., by decreasing An.

The sample was axially pumped by the frequency-doubled Nd: glass laserbeam at about six times the threshold power for Rhodamine 6G dye in thepoly(methyl methacrylate) matrix by passing the beam through the twograting reflectors tuned for 5,702 A. The output was examined using aFabry-Perot interferometer having a free spectral range of 27 gigahertzand a resolution of 0.1 gigahertz. The experimental results areillustrated in the graph of FIG. 3.

The output consisting of one strong line at 5,702 A and two weaker onesseparated by c/2nL equal to 5 gigahertz (0.05 A) where c is the speed oflight in a vacuum and n and L are defined above. Each line had a linewidth Af of less than 0.5 gigahertz (0.005 A). The total width of theoutput radiation (the three lines) was gigahertz (0.1 l A) and waslimited by the grating reflector bandwidth AA equal to 0.4 A (36gigahertz) as seen in FIG. 3. If pumped close to the threshold level, itwas clear that only one mode would oscillate. For single mode operation,it is advantageous to have one relatively narrow-band and one relativelybroad-band reflector in order that the mode not be overdetermined, thatis, in order that at least the one mode can oscillate.

Only approximately 5 percent of the pump power absorbed (1.3 megawatts)was converted into laser output (-65 kilowatts). The Stokes loss inconverting 5,300 A photons into 5,702 A photons is only about 8 percent.Much of the remaining loss was due to the small effective gratingdiameters and the less than optimum grating reflectivity 1;. Largergrating diameters and higher reflectivities would readily improve boththe threshold and efficiency of the laser considerably.

It is noted that lasers according to our invention can be quiteinsensitive to temperature and hence relatively stable under less thanideal temperature conditions. Examination of the above-described laserin a varying temperature atmosphere showed a temperature coefficient ofoutput wavelength dA /dT of less than 2 X 10 A per degree centigrade.This was due to the fact that decreases in n the refractive index of thepoly(methyl methacrylate) with temperature are almost exactlycompensated by increases in d the grating spacing with temperature. Theshift in A governed by Equation (2) above, is consequently small.

A modification of the laser described hereinabove is shown in FIG. 4.Laser 42 combines an internal axial optical guide 47 with gratingreflectors 44 and 45, all of which are illustratively induced in thephotosensitized poly(methyl methacrylate) sample 43 according to theabove-mentioned copending application.

The guide 47 is produced in the sample by sharply focusing a beam ofultraviolet radiation from a suitable source to a small beam waistwithin the desired region of the sample which is then translated in astraight line transverse to the focused beam. Upon completion of thetranslation from one lateral surface of sample 43 to the oppositelateral surface, the sample is removed from the focus. Gratingreflectors 44 and 45 are then exposed by intersecting two ultravioletbeams and causing them to irradiate the desired reflector regions spacedalong the guide. The sample is finally set aside to age.

The guide 47 in laser 42 concentrates the pump power from source 41 to arelatively small cross-sectional area throughout the sample length andthus can provide significant improvements in the gain, and hence thethreshold and efficiency of the laser. Moreover, a guided source such aslaser 42 could be readily adapted for use in an optical communicationsystem employing an integrated optics configuration. A description ofthe type of system and various circuit elements with which laser 42might be useful is contained in an article entitled Integrated Optics:An Introduction" by S. E. Miller, appearing in the Bell System TechnicalJournal, Volume 48, pages 2,059-2,069 in Sept. 1969.

Tunability of a laser according to our invention is achieved in theapparatus illustrated in FIG. 5 of the drawings. Laser 52 illustrativelyincludes stress applicators 57 and 58 which apply transverse stress tosample 53 to change the grating spacing d of reflectors 54 and 55, or tochange the resonator length L between the reflectors, or to change both.Unlike the temperature variation of the output wavelength A for thelaser, stress can produce a strain in the sample without a closelycompensating index or refraction change.

The stress may be applied uniformly in one transverse direction as shownin the drawing or may be applied in both transverse directions.Alternatively, longitudinal stress or radial stress in cylindricalsamples may be applied. In any case, a poly( methyl methacrylate) sampleis substantially elastic under strain so that tuning is reversible in aparticular range of wavelengths. In the preliminary experiment describedabove. the output wavelength A for the grating reflectors was shiftedreversibly by 11 A using transverse stress. A tuning range ofapproximately A in a suitably sized dye-doped poly(methyl methacrylate)laser would be feasible employing this means.

It should also be noted that the transverse stress may be appliedindependently to the regions of reflectors 54 and 55, or independentlyto the region between reflectors 54 and 55, or simultaneously to bothregions. The former method would change only the grating spacing d andtherefore tune the resonated wavelength A according to equation 2)above. Application of stress to the region between the reflectors wouldshift the relative modes of the laser resonator, determined by thespacing L, with respect to the grating reflectivity band. In general,both degrees of freedom in tuning are useful for a laser according toour invention in order to center the output directly under the gratingreflectivity peak as shown in FIG. 3.

We claim:

1. A device for producing the stimulated emission of radiationcomprising a sample of optically transparent material sensitized forphoto-induced molecular structural change tending to increase the indexof refraction thereof and doped with an active medium, and means forlaunching pumping radiation in said sample to excite said active medium,said sample including a pair of index grating reflectors suitable forresonating radiation at a selected wavelength therebetween to facilitatethe stimulated emission of coherent radiation at said selectedwavelength.

2. A device according to claim 1 in which said sample further includesan increased index optical guide disposed in and along the length ofsaid sample and coupled to said pair of reflectors.

3. A device according to claim 2 including means for applying stress tosaid sample to tune said grating reflectors for the selected wavelengthof coherent stimulated radiation.

4. A device for producing the stimulated emission of radiationcomprising a sample of optically transparent material sensitized forphoto-induced molecular structural change tending to increase the indexof refraction thereof and doped with an active medium providing gain ina particular wavelength range, said sample including a pair of gratingreflectors aligned and separated by a distance L within the sample, saidreflectors comprising a series of planes of alternating high and lowrefractive index with spacing d between equivalent planes suitable forreflecting radiation at a selected wavelength in said wavelength rangebidirectionally in the region between said reflectors, and means forlaunching pumping radiation in said sample to excite said active medium,whereby said reflectors provide feedback for the coherent emission ofradiation at said selected wavelength.

5. A device according to claim 4 in which the grating spacing d is givenby i d h l2n where M is the selected wavelength of coherent stimulatedradiation and n is the bulk index of refraction of said material.

6. A device according to claim 4 in which the material of said sample ispoly(methyl methacrylate) including peroxides to provide sensitivity tosaid sample and including an active dye medium.

7. A device according to claim 4 in which said sample further includesan optical guide suitable for guiding said pumping radiation and saidselected wavelength radiation within said sample, said guide comprisingan elongated region of increased refractive index disposed along thelength of said sample and coupled to said pair of reflectors.

8. A device according to claim 5 including means for applying stresstransversely to said sample in the regions of said grating reflectors tochange the spacing d between equivalent refractive index planes, wherebythe selected wavelength of coherent stimulated radiation A is tuned.

9. A device according to claim 8 in which said stressing means appliestransverse stress to said sample in the region between said gratingreflectors to change the distance L between said pair of reflectors.

10. A device according to claim 4 in which said pumping means comprisesa laser and means for directing said pumping radiation axially in andalong the length of said sample.

11. A device according to claim 4 in which said pumping means is a flashlamp and said pumping radiation is launched transversely to said samplein the region between said pair of reflectors.

12. A device for producing the stimulated emission of radiationcomprising a sample of poly(methyl methacrylate) sensitized formolecular structural change tending to increase the index of refractionthereof upon exposure to ultraviolet radiation and doped with an activedye medium, said sample including a pair of grating reflectorscomprising a series of planes of alternating high and low refractiveindex with spacing d between equivalent planes suitable for reflectingradiation of a selected wavelength bidirectionally therebetween, anoptical guide comprising an elongated region of increased refractiveindex disposed along the length of said sample and coupled to said pairof reflectors, and means for launching pumping radiation in said sampleto excite said active dye medium.

I. l i i

2. A device according to claim 1 in which said sample further includesan increased index optical guide disposed in and along the length ofsaid sample and coupled to said pair of reflectors.
 3. A deviceaccording to claim 2 including means for applying stress to said sampleto tune said grating reflectors for the selected wavelength of coherentstimulated radiation.
 4. A device for producing the stimulated emissionof radiation comprising a sample of optically transparent materialsensitized for photo-induced molecular structural change tending toincrease the index of refraction thereof and doped with an active mediumproviding gain in a particular wavelength range, said sample including apair of grating reflectors aligned and separated by a distance L withinthe sample, said reflectors comprising a series of planes of alternatinghigh and low refractive index with spacing d between equivalent planessuitable for reflecting radiation at a selected wavelength in saidwavelength range bidirectionally in the region between said reflectors,and means for launching pumping radiation in said sample to excite saidactive medium, whereby said reflectors provide feedback for the coherentemission of radiation at said selected wavelength.
 5. A device accordingto claim 4 in which the grating spacing d is given by d lambda 0/2nwhere lambda 0 is the selected wavelength of coherent stimulatedradiation and n is the bulk index of refraction of said material.
 6. Adevice according to claim 4 in which the material of said sample ispoly(methyl methacrylate) including peroxides to provide sensitivity tosaid sample and including an active dye medium.
 7. A device according toclaim 4 in which said sample further includes an optical guide suitablefor guiding said pumping radiation and said selected wavelengthradiation within said sample, said guide comprising an elongated regionof increased refractive index disposed along the length of said sampleand coupled to said pair of reflectors.
 8. A device according to claim 5including means for applying stress transversely to said sample in theregions of said grating reflectors to change the spacing d betweenequivalent refractive index planes, whereby the selected wavelength ofcoherent stimulated radiation lambda 0 is tuned.
 9. A device accordingto claim 8 in which said stressing means applies transverse stress tosaid sample in the region between said grating reflectors to change thedistance L between said pair of reflectors.
 10. A device according toclaim 4 in which said pumping means comprises a laser and means fordirecting said pumping radiation axially in and along the length of saidsample.
 11. A device according to claim 4 in which said pumping means isa flash lamp and said pumping radiation is launched transversely to saidsample in the region between said pair of reflectors.
 12. A device forproducing the stimulated emission of radiation comprising a sample ofpoly(methyl methacrylate) sensitized for molecular structural changetending to increase the index of refraction tHereof upon exposure toultraviolet radiation and doped with an active dye medium, said sampleincluding a pair of grating reflectors comprising a series of planes ofalternating high and low refractive index with spacing d betweenequivalent planes suitable for reflecting radiation of a selectedwavelength bidirectionally therebetween, an optical guide comprising anelongated region of increased refractive index disposed along the lengthof said sample and coupled to said pair of reflectors, and means forlaunching pumping radiation in said sample to excite said active dyemedium.